The invention provides a small-sized projection optical apparatus which uses a decentered prism as a projection optical system and in which some contrivances are provided to how to introduce illumination light into a reflection type display device. The projection optical apparatus comprises a reflection type display device 1, a projection optical system 2 for providing projection of an image displayed thereon and an illumination light source for illuminating a display surface. The projection optical system 2 comprises a catadioptric optical element 10 having two rotationally asymmetric curved reflecting surfaces 12, 13 having positive power. The illumination light source is located such that both a light ray leaving the display surface and arriving at a projection display screen and a light ray leaving the illumination light source and arriving at the display surface are reflected at the first and second reflecting surfaces as counted from the display surface side of the catadioptric optical element 10, and that the axis of illumination light 6 incident on the catadioptric optical element 10 is not included in a plane that passes through the axis of projection light 7 leaving the catadioptric optical element 10 and the center of the display surface.